1. Field of the Invention
This invention relates generally to electronic flash apparatus, and more particularly to an improved electronic flash unit in which light produced by the flash unit is regulated in accordance with the total quantity of scene light impinging on a photographic film in a camera.
2. Description Relative to the Prior Art
When an electronic flash unit is used in flash photography, an associated camera normally employs a flash synchronizing device which is operated in what is commonly known as "X-contact" synchronization for firing the flash unit. "X-contact" synchronization refers to firing the flash unit at a point in time which ensures that maximum brilliance of the flash is reached when a camera shutter is fully open. The maximum duration of a flash of light produced by an electronic flash unit is normally no longer than about 1 millisecond (ms), and the interval required for a camera shutter to fully open after actuation, that is, the shutter lead time, can be on the order of 4 or 5 ms. Therefore, the "X" synchronization of an electronic flash unit with a shutter must allow for the shutter lead time. Typically, X-contacts are closed to fire an electronic flash unit only after the shutter has fully opened.
A camera having a flash synchronizing device for firing a flashbulb employs what is commonly known as "M-contact" synchronization. Flashbulb light normally has a duration of approximately 40 ms, the peak light intensity being reached in approximately 10 ms followed by a gradual decay. So that the maximum brilliance of the light from a flashbulb occurs when the camera shutter is fully open, M-synchronization closes M-contacts to energize a flash firing circuit before the shutter is fully open, and often the M-contacts are closed before the shutter even starts to open.
A camera having M-contact synchronization can be adapted to fire an electronic flash unit so that the flash occurs when the shutter is open. Such a flash unit or the camera employs time-delay means associated with the M-contacts for providing a flash triggering signal a predetermined time period after a flashbulb would be fired. The delay provided by the time-delay means is related to the interval between when the M-contacts close and the shutter opens fully so that the electronic flash unit can be fired when the shutter is fully open. U.S. Pat. Nos. 3,521,540, 3,559,548 and 3,969,737 disclose electronic flash units which may be operated with cameras adapted to fire flashbulbs through M-contacts.
It is known for electronic flash units to employ light-integrating circuits for controlling the amount of flash light in accordance with reflected scene light. Such flash units are known as quench type or quenchable electronic flash units. A quenchable electronic flash unit would include an energizable quench circuit that is coupled to the light-integrating circuit and that is also associated with a flashtube and a flash-firing capacitor. When the light-integrating circuit senses a predetermined amount of reflected scene light, representing a desired exposure, the quench circuit is energized. When this happens, further transfer of energy from the flash-firing capacitor to the flashtube is terminated, thereby quenching the flashtube.
A quenchable electronic flash unit normally includes apparatus for preventing the light-integrating circuit from responding to any light prior to the actual firing of the flashtube. Such apparatus is considered to be highly desirable because, if the light-integrating circuit were to respond to light other than that light produced by the flash unit itself, the light-integrating circuit would prematurely energize the quench circuit causing early flashtube quenching or, in some situations, causing the flashtube not to fire at all.
A variety of patents disclose quenchable electronic flash units having means for preventing the light-integrating circuit from responding to light prior to the production of light by the flash unit. For example, U.S. Pat. No. Re 26,999 discloses a quenchable electronic flash unit in which a light-integrating circuit is energized only after the flashtube has emitted a pulse of light.
U.S. Pat. No. 3,519,879 includes a light-integrating circuit for a quenchable electronic flash unit wherein the triggering of the flash unit is the sole condition which energizes the light-integrating circuit. To accomplish this, a normally conducting transistor de-energizes the light integrating circuit. This transistor is turned OFF, to energize the integrating circuit, only upon the occurrence of a voltage change, in a flashtube trigger circuit, representative of the triggering of the flashtube.
U.S. Pat. No. 3,727,100 discloses a quenchable electronic flash in which a delay circuit is provided for causing the integrating circuit to be ready to receive reflected scene light about 10 microseconds prior to the production of light by the flashtube. This is done to insure that a photoconductive element of the integrating circuit is in a stable electric condition at the instant the flashtube emits light, thus permitting the photoconductor to more rapidly respond to reflected flashtube light.
Accordingly, these prior art quenchable electronic flash units commence to measure light reflected from an object being photographed only from the instant of the production of the flash light or a very brief interval prior to such production. When such electronic flash units are used with cameras employing X-contact synchronization or are used with cameras employing M-contact synchronization with time delay means associated with the M-contacts, the flash unit light-integrating circuit does not respond to light until after the camera shutter opens.
Frequently, however, it is desirable to provide flash illumination when there exists significant ambient illumination, such as operation of a photographic camera in what is commonly called a "fill-flash" mode. These prior art flash units have a disadvantage in fill-flash because the flash integrating circuit does not take into account the light "seen" by photographic film in the camera during the interval the camera shutter is opened prior to the production of flash light. The initial exposure, which occurs during this interval, can be a substantial portion of the total exposure. Unless this initial exposure is effectively accounted for by the light-integrating circuitry in the flash unit, an appreciable over-exposure error results in fill-flash, because the flashtube is not quenched soon enough. The error can be particularly troublesome for a photographic process that has a narrow "exposure latitude" such as used in self-processing cameras.